1. Field of the Invention
The present invention relates to a spatial light modulator element used for projection display.
2. Description of the Prior Art
There are three approaches to development of projection display. In these three approaches, the projection display with the use of a Braun tube has a limit in brightness of the Braun tube and a fault that a device becomes large-sized, thereby limiting enlargement of display area. On the other hand, in case of the liquid crystal projection display consisting of a liquid crystal panel having thin film transistor array, there are problems such as insufficient resolution of the liquid crystal panel, small pixel opening ratio of the liquid crystal panel, photocurrent generation by irradiation of light in the thin film transistor array consisting of an amorphous silicon thin film, and the like.
Therefore, there is hopefully expected a projection display assembling a spatial light modulator, a small display and a light source in an enlarged projection optical system. This system once writes a weak image in the spatial light modulator, modulates another read-out light according to this write-in information, and projects the image on a screen.
In a display of this type, the so-called liquid crystal light valve with the use of a nematic liquid crystal for a light modulation layer is common In this system, however, one of P and S polarized light components in read-out light modulated by a liquid crystal layer passes through a polarization beam splitter and is projected on a screen. Therefore, when the read-out light is unpolarized, more than 50% of the light is absorbed by the polarizing beam splitter. As a result, the power of the read-out light projected on the screen becomes low and the beam splitter generates heat.
As a method for solving this problem, there has recently been proposed a new liquid crystal light valve with the aid of a light scattering phenomenon of a polymer-dispersed liquid crystal (PDLC). The PDLC is composite material dispersing liquid crystal droplets into a transparent polymer film such as acryl and the like, and its study has started only several years ago. A liquid crystal light valve of the new type modulates the read-out light incident on the light valve by irradiating a write-in light on the another side of the light valve without a polarization beam splitter. Therefore, it can be expected that the most part of the read-out light can be utilized as a projection light. Moreover, there is no need to provide alignment layers on both side surfaces of a liquid crystal layer, so that it is easy to manufacture a spatial light modulator of a large area. Moreover, the light valve does not modulate the phase of read-out light, so that even if a thickness of the liquid crystal layer is uneven, there is no serious influence upon the spatial uniformity of the read-out light.
However, even in case of using such spatial light modulator, the read-out light is irradiated to a light modulation layer consisting of PDLC. Furthermore, a write-in light is irradiated into a photoconductive layer from the side surface opposite to the read-out light. Therefore, it is necessary to reflect the read-out light between the light modulation layer and the photoconductive layer and to prevent the read-out light from leaking to the photoconductive layer.
For this purpose, it has been known to provide a dielectric multilayered film mirror between the photo-conductive layer and the light modulation layer consisting of PDLC. However, the dielectric multilayered film is made by stacking two kinds of dielectric films having high or low refractive index, and its reflectance is about 90-99% at the most. The read-out light has far greater intensity than the write-in light, so that if a small part of the read-out light leaks to the side of the photoconductive layer, the photoconductive layer is sensitized and its resistance is lowered. Therefore, in order to improve an intensity ratio (amplification ratio) of the read-out light to the write-in light, it is necessary to isolate effectively isolate the read-out light passed through the dielectric multilayered film between the photoconductive layer and the light modulation layer.
A task of the present invention is to intercept effectively the read-out light between a photoconductive layer and a light modulation layer in a spatial light modulator, which is provided with a transparent electrode and a light modulation layer which consists of a liquid crystal material for modulating the intensity, phase or running direction of read-out light by applying a voltage.